A toner composition consists of toner particles and any additives carried by the toner particles, in order for example to improve the properties of the toner particles. Usually, the additives are present in the form of particles that are small compared to the toner particles.
In this context, the term “developer” means a powder that comprises a mixture of toner particles and carrier particles. The carrier particles are large compared to the toner particles and carry the same. During development, the toner particles are applied onto a device, preferably a printer drum, the surface of which has electrically highly charged regions and electrically un- or low-charged regions. The toner particles adhere to one of the regions and the thus “developed” image is transferred to a clear glass plate. A masking pattern is formed thereon in order to mask a corresponding region of a layer of build powder particles that are to bond to each other by radiation passing through the mask. The build powder particles that have been exposed to radiation are bonded to each other by e.g. sintering or melting together. When the volume body has been built up, the build powder particles that have not been exposed to radiation are removed and are optionally reused.
It is known to produce prototypes from originals in the form of layered CAD drawings, wherein the information from each layer is used to sinter together or melt material, then allowed to solidify into a three-dimensional body. In case a computer controlled laser beam is used to selectively sinter/melt the material, expensive machines are required and the building up of a normal size prototype is time-consuming. For some applications, it has been possible to increase the speed by using an UV radiator that illuminates the entire surface simultaneously through a masking pattern of toner on a glass plate, but such machines are also expensive to acquire.
EP 1,015,214, which corresponds to U.S. Pat. No. 6,531,086, discloses a method and a device for the production of volume bodies from a large number of mutually connected layers of a particulate material, such as a powder, wherein the information concerning the appearance of each layer is obtained from the CAD unit or the like of a computer. The described method and device enable the formation of a three-dimensional body, based on originals in the form of layered CAD drawings, wherein the information from each layer is used to sinter or melt together powder material. In particular, said method comprises applying an essentially even layer of particles on a substrate, placing a masking pattern according to information from the CAD unit on a masking device that is brought over and close to the particle layer, and placing or passing a radiator, preferably an IR radiator, over the masking device, whereby particles that are not covered by the masking pattern are exposed to radiation and are thereby bonded together. Thereafter, the masking pattern is removed from the masking device and new steps according to the above, with reuse of the toner powder, are performed until the volume body has been produced. A device for performing this method is cheaper than the above described devices. Furthermore, the production of a layer will be very fast, as the entire surface or a large part of the surface can be irradiated simultaneously. The masking device comprises e.g. a transparent quartz plate, on which for example a printer head has deposited a masking pattern of a suitable toner that does not adhere onto the quartz plate when melting.
As an example of a suitable toner powder that does not melt bond, aluminum oxide is suggested, which has a high melting point and a high degree of reflection. However, it has been shown that aluminum oxide degrades the equipment and that its colour, white to ivory, does not give a desired high reflection. Direct (i.e. specular) reflection does not occur from aluminum oxide and diffuse reflection inside the powder layer is low. Accordingly, the radiation does not spread within the toner layer in order to give it a better chance of hitting absorbing sites. Hence, the use of aluminum oxide as a toner will result in the risk of emergence of areas of higher temperature in the mask, i.e. so called “hot spots”.
For radiation incident on a masking pattern that partly passes to the powder particles, from which the volume body is to be built, the following equation applies: transmission+reflection+absorption=100%. The toner of the masking pattern performs its optical function in powder form, as opposed to a “regular” toner that is melted onto its substrate. Thereby, the powdered toner layer acts as an optical diffuser, which means that the portion of the total incident electromagnetic radiation that is not absorbed will be transmitted and reflected in several directions. In other words, the total transmission consists of direct transmission as well as diffuse transmission, just as the total reflection consists of direct reflection and diffuse reflection. Since the aluminum oxide of the known method has a particularly bright colour, a relatively large portion of the radiation will be reflected both directly and diffusely, while a relatively low portion will be absorbed.
The effective transmission is of interest for the masking action of the toner layer, as it corresponds to the exposure, to which a masked point on the build powder surface is subjected when the build powder grains that are to be melted together are directly irradiated in order to build up the volume body. This value includes directly and diffusely transmitted light. The radiation that does not penetrate the mask has either been absorbed or reflected. The largest part of the reflected radiation is diffusely reflected and should be stopped from reaching the build powder and thus adversely affecting it such that the powder grains, which were intended to be protected by the mask, stick together to form aggregates that result in problems in connection with the continued building up of the volume body.
The present invention is directed, at least in part, to improving or overcoming one or more aspects of prior toner compositions for forming a masking pattern adapted to be used in a method of building up a three-dimensional body from a large number of mutually connected layers of powder particles that are bonded to each other using radiant heat.